In a cellular communication system, such as the Global System for Mobile communication (GSM), each of the subscriber units (typically mobile stations) communicates with typically a fixed base station. Communication from the subscriber unit to the base station is known as uplink and communication from the base station to the subscriber unit is known as downlink. The total coverage area of the system is divided into a number of separate cells, each predominantly covered by a single base station. The cells are typically geographically distinct with an overlapping coverage area with neighbouring cells. FIG. 1 illustrates a cellular communication system 100. In the system, a base station 101 communicates with a number of subscriber units 103 over radio channels 105. In the cellular system, the base station 101 covers users within a certain geographical area 107, whereas other geographical areas 109, 111 are covered by other base stations 113, 115. Some overlap areas 117 can be covered by more than one cell.
As a subscriber unit moves from the coverage area of one cell to the coverage area of another cell, the communication link will change from being between the subscriber unit and the base station of the first cell, to being between the subscriber unit and the base station of the second cell. This is known as a handover. Specifically, some cells may lie completely within the coverage of other larger cells.
All base stations are interconnected by a fixed network. This fixed network comprises communication lines, switches, interfaces to other communication networks and various controllers required for operating the network. A call from a subscriber unit is routed through the fixed network to the destination specific for this call. If the call is between two subscriber units of the same communication system the call will be routed through the fixed network to the base station of the cell in which the other subscriber unit currently is. A connection is thus established between the two serving cells through the fixed network. Alternatively, if the call is between a subscriber unit and a telephone connected to the Public Switched Telephone Network (PSTN) the call is routed from the serving base station to the interface between the cellular mobile communication system and the PSTN. It is then routed from the interface to the telephone by the PSTN.
Users of subscriber units such as mobile phones, mobile terminals, user equipment etc. typically either pay for their calls in advance (known as pre-paid) or are billed at regular intervals, typically on a monthly basis. The monthly charges typically comprise a line rental paid in advance as well as call charges for any calls made in the previous month. The subscriber is provided with an itemised bill or invoice showing the duration, cost and phone number for each call made. The subscriber may for example use this information to check the accuracy of the invoice, verify that no unauthorised calls have been made or categorise the charges for example into private and business calls.
A problem with conventional invoices or bills is that it is time consuming for the subscriber to identify calls made from the information provided. Most people find it difficult to remember phone numbers and therefore checking an itemised invoice becomes very cumbersome and time consuming. There is therefore a need for a billing system providing improved information to the subscriber.